This invention relates to apparatus for forming metal by drawing. More particularly, the invention is for a blankholder portion of a draw press which enables pressure to be differentially applied over the flange portion of a workpiece to minimize or eliminate earing of the drawing object.
A typical way to make a drawn cup is to force a flat metal blank through a die with a punch. As the shape of the blank is changed from a planar form to a cylindrical shape having a reduced diameter, the metal is stressed in tension in a radial direction and stressed in compression circumferentially. Depending upon a number of factors, such as the severity of the draw, the thickness of the blank, and forming characteristics of the metal, the blank may wrinkle in the flange adjacent to the die opening to an extent that an uneven or objectionable appearance is created in the cylinder sidewall after drawing. Another problem which may arise in drawing is earing which is the forming of tabs or ears which project upwardly from the edge of the open mouth of the drawn cylinder. Earing, at least in part, is a function of the anistropic character or nature of the metal. Anistropy is the exhibition of properties in a material of different values when measured along axes in different directions, or it is the exhibition of different reactions along different axes in response to application of external stimuli. Thus, when a circular metal blank is drawn, the response of the blank in the direction of rolling may be different than at an angle to the direction of rolling. Whether there is a difference, or the extent of the difference, depends upon the anisotropic properties of the metal which, in turn, depends upon the composition and temper of the metal and fabricating practice in making the sheet. If the metal is anisotropic, the blank flange will develop thickened portions along predictable radial lines as the blank is compressively stressed in being drawn into the die. In drawing, the clearance between the punch and the die is such that there is substantially no reduction in thickness in any portion of the sidewall of the cup as it is drawn, and the thickened portions will be substantially parallel in the drawn cup sidewall. The number of thickened portions and the degree of thickening is a function of the anisotropic properties of the metal and the severity of the draw. In addition to the metal being nonuniform in thickness circumferentially, it varies in thickness from top to bottom of the can; i.e., the sidewall increases in thickness from the bottom to the top. Earing is directly related to the anisotropic characteristics of the metal and its tendency to generate thickened portions during drawing. The ears or tabs are extensions beyond the rim of the cup of the thinner portions between the thickened portions. The ears generally occur in groups of 4 or 8 with the peaks of the projections located at 45 degrees and/or at 0 and 90 degrees to the rolling direction. The number of ears is a function of the anisotropic characteristics of the metal. Earing is undesirable for a number of reasons, such as excess scrap generation in trimming, and breaking off the ears, and thus generating a risk of damage to the workpiece or tooling.
The use of pressure pads or blankholders around the die opening can minimize or eliminate the formation of wrinkles. Thus, if the portion of the tooling around the periphery of the die opening is adapted to squeeze or clamp the blank flange with sufficient pressure to hold it flat without reducing it in thickness while permitting it to slide radially inwardly into the die opening, a drawn cylinder can be formed without encountering the problems created by wrinkling. The effectiveness of such clamping has its limitations, however. It may not be effective in preventing thickening, or the pressure required to prevent wrinkling or thickening may be so great as to prevent the blank from being drawn into the die and, as a result, the blank will fracture.
A number of solutions to the problem of overcoming wrinkling and earing in deep drawn articles have been proposed. Beck U.S. Pat. No. 3,349,153 discusses deep drawing thin films or foil. The blank is held between clamping jaws having a plurality of concentric grooves in one or both of the jaws. The grooves are connected to a compressed air source, and as pressure is applied to the punch to force the blank through the die, the air pressure to the grooves is increased until the jaws are forced apart sufficient to permit the film or foil to slide. Thus, the clamping pressure is balanced against the drawing force, and as the blank is drawn inwardly, less and less clamping pressure is required. By providing a mechanism for varying the clamping force during the draw cycle, fracture of the cylindrical article is prevented. Meyers U.S. Pat. No. 3,420,089 is concerned with making cylindrical shapes having domed-shaped ends. According to Meyers, at least a part of the problem with prior drawing apparatus is that the blankholder or draw ring is adapted in a manner that pressure is applied to the blank unevenly and, as a consequence, the metal forms unevenly. To overcome uneven application of pressure, Meyers provides a blankholder which has three concentric rings, each of which is connected to separate fluid pressure sources. Thus, a different pressure can be applied uniformly to each portion of the blank in contact with the rings. The innermost ring supplies the highest unit pressure since the greatest pressure is needed there to prevent wrinkling. Saunders U.S. Pat. No. 3,494,169 also attributes wrinkling and earing problems to a nonuniform application of pressure. Saunders states that with the use of conventional tooling, the unit pressure on the blank increases as the blank is drawn because the force imposed with the blankholder is distributed over an ever-decreasing area of the blank. This may result in creating a tensile stress sufficient to thin the metal as it is drawn. Saunders provides a blankholder having independently movable concentric rings. The rings sequentially apply pressure to the blank as it is drawn inwardly, and thus a uniform pressure is maintained on the blank throughout the drawing cycle.
The foregoing are but a few of the many patents directed to solving the problem of preventing wrinkling and earing in making deep drawn articles. Blankholders having concentric rings capable of applying varying levels of force to the flange are satisfactory for controlling wrinkles and can also be effective in preventing earing. However, the rings may be subject to deflection or cocking out of a plane parallel to the flange resulting in a distribution of force on the flange which does not insure thinning of all of the thickened portions and thus eliminate earing.